Before the discovery of antibiotics, community-acquired infections were a major threat to the health and welfare of people in the United States, and they continue to be a major problem in developing countries. However, soon after the discovery of penicillin and wide spread access to antibiotics in the 1940's, bacteria began to develop varied degrees of resistance to these drugs. While new drugs have been introduced since the discovery of penicillin, the majority of them are the result of varied combinations of substituents on one of about 9 molecular scaffolds. There should be no surprise that the number of microbes developing resistance is growing rapidly, and that their resistance mechanisms are becoming more sophisticated. For example, antibiotic resistance was initially a problem associated with nosocomial infections. But now, there has been an increase in occurrences in community acquired cases. Antibiotic resistance appears to now threaten the utility of “last resort” drugs, such as vancomycin, the drug of choice for treating methicilin- and multidrug-resistant Staphylococcus aureus infections. The problem of antibiotic resistance is further complicated by the threat of bioterrorism, and the potential use of pathogens that have been intentionally altered in order to enhance their resistance to antibiotics and to enhance their virulence or lethality.
Today, gastrointestinal pathogens continue to present a threat to the health of Americans and people worldwide, particularly children in developing nations. While many of these illnesses can be treated, several of them (such as shigellosis and cholera) can have fatal consequences if untreated. There is an urgent need to develop new therapeutics to better address this threat. It has been estimated that as many as about 325,000 Americans are hospitalized and up to about 5,000 die per year due to food-borne pathogens and the resulting gastrointestinal (GI) infections.
The Shigella family of bacteria is a particularly virulent group of gastrointestinal pathogens that are spread by contaminated food or water. Infection can occur with as few as 10 ingested cells. According to the Centers for Disease Control, as many at 18,000 cases of shigellosis are reported in the United States each year, and it is estimated that the actual number of cases in the United States is closer to 300,000 per year. Shigellosis is a greater problem in developing countries, accounting for about 99% of the estimated 165 million cases worldwide each year. Children in developing countries are particularly susceptible to shigellosis, with children under five accounting for nearly 60% of the ˜1.1 million deaths each year.
In 1994, there was an outbreak of Salmonella enteriditis in the United States that affected approximately 224,000 people. This outbreak was traced to a tanker shipment of contaminated liquid ice cream.
An outbreak of Salmonella typhimurium in Illinois in 1985 affected over 170,000 people as a result of contaminated milk. What made this outbreak particularly disturbing was the fact that the strain of S. typhimurium involved demonstrated resistance to nine different antibiotics.
There are other known pathogens of greatest concern. For instance, the associated illness for pathogen Salmonella typhi is acute fever, diarrhea and potential intestinal rupture. The associated illness for pathogen Shigella dysenteriae is dysentery, with a fatality rate of up to about 20%. The associated illness pathogen Escherichia coli (E. coli) (O157:H7) is acute hemorrhagic diarrhea and possible long-term problems. The associated illness pathogen for Vibrio cholerae is severe diarrhea, with up to about 50% fatality rate.
A variety of strategies have been reported for the targeted delivery of antimicrobial agents to bacteria. In creating antibiotics for such pathogens, compounds were selected based on their ability to preferentially affect systems or features that are unique to microbe(s). They have also been selected based on being sufficiently different from analogous systems or features in host cells. However, many compounds that show very potent antimicrobial activity tend not to be suitable for use as therapeutics due to undesirable side effects or poor selective toxicity. Targeted delivery is expected to allow the use of alternative antimicrobial agents, such as nitric oxide (NO), that are less specific in the types of cells they affect and may even have broader impact. Yet, if delivered nonspecifically, undesirable side-effects may result. Furthermore, while a diverse range of strategies for targeted delivery of therapeutics have been explored for the selective delivery of chemotherapeutics to cancerous cells, they have only recently been investigated for the treatment of infectious disease.
For example, constructs based on chlorine6 conjugated to polylysines of varied lengths and varied degrees of substitution have been investigated against representative gram-negative and gram-positive bacteria for the intracellular delivery of the photosensitizer (chlorine6). In these studies, cells were treated with the conjugate and then exposed to 660 nm light, which triggers the generation of singlet oxygen and free radicals leading to cell death. While useful in localized applications, polylysine conjugates are generally not well suited for systemic administration. They tend to provide limited specificity in their delivery of attached drug moieties by entering host cells, as well as invading microbes.
Similar polylysine peptides have been used for the transduction of proteins across the membranes of mammalian cells.
Even liposomes have been explored for the intracellular delivery of aminoglycoside antibiotics. It has been reported that the liposome-encapsulated aminoglycosides demonstrated significantly improved potency over the corresponding free drugs, when evaluated against Pseudomonas aeruginosa for the treatment of pulmonary infections.
Additionally, filamentous phage has been used for the targeted delivery of chloramphenicol as a model antibiotic. These phage-based constructs incorporate a filamentous phage that had been selected from a phage library for specific binding to S. aureus. As anticipated, chloramphenicol-phage conjugates with 2,000-4,000 drug molecules/phage retarded S. aureus growth, while comparable concentrations of free chloramphenicol had no significant impact on growth. As with antibodies, the specificity of phage-derived peptides makes them very specific for the target pathogen. However, at the same time, their specificity may also prevent their utility with organisms other than the one for which they were designed.
Thus, there is an urgent need to develop new antibiotics and approaches for treating infections.